There are three control functions each of which may or may not be required in a particular radar transponder application.
First, a transponder may or may not be required to respond to only one of a plurality of time coded input pulses. Second, a transponder may or may not be required to respond by transmitting one of a plurality of time coded pulse signal combinations, and third, there probably will be a requirement for a transponder to be limited in its response to radar interrogation to prevent self destruction due to operation of the transponder transmitter at excessive duty cycle rates.
In the past, one or more of the aforementioned three functions have been supplied in a given transponder as a result of the requirements for a specific transponder application. Thus, each transponder was "tailored", or custom designed, for the intended application. High cost and the sheer bulk and high power requirements of circuits able to perform all of the possible combinations of the three functions have prevented manufacturers from marketing a universal system; that is, a system able to provide any possible combination of all three functions.
Prior art equipment providing any of the decode, encode or over-interrogation control functions used analog circuit techniques that required resistors or capacitors, selected or adjusted during assembly or test. Code space timing signals have been generated by monostable multivibrators utilizing resistor-capacitor timing circuits, passive delay lines or oscillators (utilizing inductor-capacitor timing) synchronized to the incoming video interrogating pulses. Over-interrogation detectors used in prior art control functions have been of the resistor capacitor integrator type. Each transponder transmission placed a fixed electrical charge on a capacitor. The capacitor discharge rate was controlled by the discharge resistance value to determine the over-interrogation threshold rate. Detection depends upon sensing a predetermined threshold voltage level on the capacitor.
Since the present practice of custom design for each application prevents manufacturers from producing a sizable backlog of inventory, there is, typically, a relatively long delay from the time specifications are determined until units may be delivered to meet these specifications for a particular application. The manufacturer is also limited in a given production run to transponder units meeting specific requirements. This limits the production quantity which has the secondary effect of increasing costs.